The present invention relates to a multivibrator waveform monitoring system, and, more particularly, to a multivibrator waveform monitoring system which is used for a geological survey for carrying out a geological survey by giving vibration to a ground surface (strata) by using a plurality of synchronized seismic energy sources and analyzing reflected vibrations from the ground based on the vibrations applied.
In recent years, for carrying out a geological survey such as an oil resource survey or the like within a few kilometer range, a method using oil-pressure control vibrators of seismic energy sources have been widely used to make a non-explosive seismic center, instead of using an artificial seismic prospecting method using dynamites as a seismic center.
One of the seismic energy sources using an oil pressure control type vibrator, VIBROSEIS (a registered trade mark of CONOCO, INC.) is available which was developed by CONOCO, INC. in the USA. Unlike a pulse type seismic energy source for generating vibrations of a single frequency, VIBROSEIS continues to send vibrations (called a sweep) of which oscillation frequency is continuously changed, to underground for a few seconds to several tens of seconds. By sweeping the vibrations, a time of the transmission of vibrations from the seismic center to the point of receiving the vibrations is calculated in the same manner as is done with the radar. Reflected vibrations are received by using a seismometer installed on the ground surface or within a mine. These reflective vibrations are the result of superposition of reflected waves and refracted waves (signal component) from a reflection surface with other vibrations. Therefore, a sweep signal is used as a reference of vibrations to be sent to the underground, and a correlation function is obtained between the seismic waves (reflected vibrations) obtained and the sweep signal, to thereby take out signal components (the reflected waves and the refracted waves) mixed with noise components from the seismic waves.
FIG. 9 shows a block diagram of the structure of the VIBROSEIS system. The system includes a vibrator loaded on a vehicle, a recording truck and field geophones.
The recording truck has an encoder sweep generator, and the vibrator has a vibrator control circuit for storing a reference signal generator for generating a signal similar to that of the encoder sweep generator. The encoder sweep generator of the recording truck transmits a starting signal to the vibrator control circuit through the radio, and the vibrator control circuit starts the sweep reference signal of the generator in response to the starting signal. The waveform of the sweep signal is set to be in the same phase so that of the waveform of the encoder sweep of the recording track as described in detail later. The vibrator control circuit drives the vibrator by using the sweep waveform as a reference signal.
The waveform of the sweep signal of the encoder sweep generator is stored in the storage unit, and the reflected vibration received by the field geophone section is A/D converted by the analog module and the result is stored as digital data in the storage unit. As described above, a correlation function is obtained from the sweep signal stored in the storage unit and the data obtained from the field geophone, so that the reflected waves are calculated.
FIG. 10 shows a conceptional diagram of the vibrator of this system. The vibrator is loaded on a vehicle so that the vibrator can be moved. The portion of the plate which is in direct contact with the ground surface is called a base plate, which is pressed against the ground surface by the vehicle through an isolation bag. A piston directly connected to the base plate pierces through a cylinder called a reaction mass to alternately increase the oil pressure at arrowed portions in the diagram, to thereby generate vibrations.
FIG. 11 shows a conceptional diagram of the oil pressure control of the vibrator. The oil pressure mechanism is a two-stage amplification type as follows. The vibrator control circuit explained in relation to FIG. 5 controls the current of a torque motor based on the sweep signal of the reference signal generator, to operate a first piston called a pilot stage, with a result that a second piston called a main stage is operated, to apply an oil pressure to the cylinder within the reaction mass. The cylinder is moved vertically to vibrate the base plate.
The vibrator is phase controlled as follows. As described above, the vibrator generates vibrations (sweep) by changing the oscillation frequency in accordance with the sweep reference signal of the reference signal generator. Since the vibrator is driven by a mechanical structure using oil pressure, a phase difference is generated between the vibrations and the reference signal when the oil pressure unit is directly controlled with the sweep reference signal. Further, a resonance frequency which depends on the weight of the base plate and the Poisson's ratio, etc. exists on the ground surface. Accordingly, the resonance frequency is different depending on the place where the vibrator is installed, and the phase deviation attributable to this difference is also applied. To eliminate the above deviations, an acceleratometer is provided on the base plate to detect the vibrations of the base plate. This signal is fed back to the reference signal, to thereby compensate the phase difference between the reference signal and the vibrator vibrations. The reference signal applied with this feedback is used to control the vibrations of the vibrator.
According to the geological survey based on the seismic energy source using the above-described vibrator, vibration energy per one unit seismic energy source applied to the ground surface is small unlike the energy applied in the geological survey using an explosive. Therefore, in the geological survey of deep layer portions requiring large energy, a plurality of seismic energy sources are operated simultaneously.
When a plurality of seismic energy sources are to be used, it is required to vibrate these seismic energy sources accurately in the same phase in order to increase the accuracy of the measurement. In the VIBROSEIS system, the waveform of the sweep signal of the reference signal generator of the vibrator control circuit for each seismic energy source is started at the same time by the starting signal (operation starting instruction) from the recording car so that sweep waveforms of the same phase are generated by all the generators, as described above. With the above arrangement, the vibrators of all the seismic energy sources are designed to be operated to generate vibrations in the same phase.
However, the above system is of the so-called independent synchronization type, which makes it difficult to synchronize all the vibrators accurately. As a result, phase differences occur in the vibrations of the vibrators. Therefore, it has been difficult to carry out a geological survey very accurately because of the mutual interference of vibrations. Particularly, it is not possible to curry out the feedback between the seismic energy sources, thus causing a problem that the phase differences become larger as the time passes since the start of the measurement, even if the phase differences are small at the start.
A similarity test is carried out when it is necessary to confirm that the sweep generated by the vibrator is in synchronism with the reference signal. This test is carried out for each seismic energy source by sending back a signal of the accelerometer, fitted to the vibrator before or after of a measurement, to the recording truck by the radio or a cable system, and comparing the sent-back signal with a signal generated by the encoder sweep generator of the recording truck. With this test, it is only confirmed, before or after the measurement, that the a deviation of synchronization is within a permissible range or not. Therefore, it is not possible to confirm whether the signals are being correctly synchronized or not during the measuring period, so that it is difficult to decide whether the data collected are correct or not. Therefore, even if the phase of the vibrations of a certain unit (source) deviates to a large extent from the phase of the variations of other units, the measuring is continued as it is.
According to this system, a signal of the encoder sweep generator is stored in the recording truck, and a correlation function is obtained between this stored signal and a reflected seismic signal obtained, to thereby take out the original signal components (reflected wave and refracted wave) for carrying out an analysis of geology, as described above. However, as the phase difference exists between the signal of the encoder sweep generator and the vibration of the seismic energy source, it is not possible to take out the original signal components accurately by obtaining a correlation function. From this aspect as well, it has been difficult to carry out an accurate measurement.